Prior co-pending application Ser. No. 337,085, filed Apr. 12, 1989 and application Ser. No. 416,148 filed Oct. 2, 1989, disclose the preparation of N,N-dimethylethylamine alane and N-methylpyrrolidine alane by extraction of AlH.sub.3 from lithium aluminum tetrahydride by the appropriate tertiary amine. Use of amine alanes as reducing agents is also disclosed therein.
Amine alanes have also been produced by the reaction of alkali metal aluminum tetrahydride, silicon tetrachloride, and tertiary amine; by the reaction of alkali metal aluminum tetrahydride, tertiary amine, and hydrochloric acid or an analog; and by the reaction of alkali metal aluminum tetrahalide, alkali metal aluminum tetrahydride, and tertiary amine. These methods are disclosed in the following U.S. Patents, respectively: Marlett and Frey, U.S. Pat. No. 4,757,154; Marlett, U.S. Pat. No. 4,748,260; and Marlett, U.S. Pat. No. 4,665,207. These patents also make reference to Marlett, U.S. Pat. No. 4,474,743, and to references cited therein, which set forth several other general methods for preparation of amine alanes: ##STR1##
Also, LiAlH.sub.4 can be reacted with a trialkyl amine. HCl complex to precipitate LiCl and form AlH.sub.3.NR.sub.3 where R is alkyl.
U.S. Pat. No. 4,474,743 also states that: "Alane, that is, aluminum trihydride or AlH.sub.3, has in the past been produced from the reaction of LiAlH.sub.4 and AlCl.sub.3 in ethers. Also known is the production of an alane-dimethyl ether solution from the reaction of LiH and AlCl.sub.3 in dimethyl ether, catalyzed by NaAlH.sub.4."
Brown and Yoon, J. Am. Chem. Soc. 1966, 88, 1464-1472, have prepared alane in tetrahydrofuran, that is, AlH.sub.3 in THF, by reaction of exact stoichiometric quantities of lithium aluminum tetrahydride and 100% sulfuric acid. After about three days, the resulting solution begins to decompose with cleavage of the solvent.
Brown and Yoon, cited above, and Yoon and Brown, J. Am. Chem. Soc. 1968, 90, 2927-2938, have reported the use of this reagent, aluminum hydride in THF, for selective reductions of organic compounds. They made comparisons of this reagent with LiAlH.sub.4 and with alkoxy-substituted lithium aluminum hydrides and found "significant differences in reducing characteristics" (Yoon and Brown, p. 2927, column 1).
A selective reduction process is useful in organic synthesis where more than one functional group occurs in a molecule and it is desired to reduce one but not another, for example, to reduce an acid or a nitrile without reducing a nearby halogen-containing functionality or double bond.